The invention relates to a sintered glass-ceramic structure layer having holes filled with conductive material and a method of making such a structure where a green sheet is formed of a mixture containing glass particles and a binder which is cast in a thin layer and dried, the holes are subsequently formed and filled with a conductive paste and finally the green sheet is heated to the sintering temperature of the glass-ceramics.
Glass-ceramic structures or substrates of this kind where however always several green sheets are laminated together prior to sintering, and the method for forming the same are described in U.S. Pat. Nos. 4,301,324 and 4,234,367 (whose teachings are incorporated herein by reference thereto). Sintering in this context means the heating of the substrate where the binder is removed, the glass particles coalesce, crystallize and optionally recrystallize in order to be converted into the most stable crystalline modification with the sintering temperature being the highest applied temperature. The glass consists of .beta.-spodumene being composed essentially of Li.sub.2. O A1.sub.2 O.sub.3 . 4 SiO.sub.2 and cordierite composed essentially of 2 MgO . 2 Al.sub.2 O.sub.3 . 4 SiO.sub.2 glass. The conductive material in these prior art substrates consists of copper that forms the vias and also the conductive patterns on the inner layers of the substrate. With the combination of these materials, i.e. copper and glass-ceramics as used in the prior art, the production of a hermetically sealed substrate was not possible in the sintering process. The mechanical tensions between the copper conductors and the glass-ceramics destroy each connection between these two materials. Therefore in the prior art process provisions are made for not filling completely the via holes with copper and to avoid a connection between the glass-ceramics and the copper vias, which is accomplished by cooling the substrate after sintering at a very small oxygen partial pressure. By that any chemical oxygen bridge connection formed eventually during sintering between the copper metallurgy and the glass-ceramic are decomposed, so that the copper with its coefficient of thermal extension of 17.times.10.sup.-6 and the glass-ceramics with its coefficient of thermal extension of 3.0.times.10.sup.-6 are separated from each other. In subsequent heating processes, the copper in the substrate can dilate and contract without mechanical tensions being created between the copper and the glass-ceramic. The considerable disadvantage of this process is the formation of a gap between the copper vias and the glass-ceramic because these gaps allow liquids used in subsequent processes to penetrate into the substrates causing the copper metallurgy to corrode or to make the substrate non-functional. To cure this problem a repair process was installed by which after sintering the gaps are filled with an elastic material, like polyimide.
A method described in the article of R. C. O'Handly with the title "Near-Eutectic Alloy Conductors for Glass-Ceramic-Based MLC" published in the IBM Technical Disclosure Bulletin, Volume 23, No. 12, May 1981, page 5593, tried to overcome the problems encountered in confirming dissimilar materials. The author proposes the replacement of the copper containing paste by one containing PdSi, PdP or NiP alloys. Apparently these alloys help to overcome at least partly the thermal expansion problem because they have a melting point at a temperature within the sintering temperature range. In another article with the title "Metallurgy for Multilayer Ceramic Modules" published in the IBM Technical Disclosure Bulletin, Volume 18, No. 9, Feb. 1976, page 2856, J. J. Knireck inter alia describes a method to improve inter alia the thermal coefficient of expansion match between the conductor material and the alumina ceramics in conventional multilayer ceramic modules by using Pd/Mo or Pt/Mo/Pd alloys instead of molybdenum. The addition of the Pd to Pt/Mo produces liquid phase sintering. This method reduces the risk of cracking.
It is one object of the invention to provide a glass-ceramic structure having hermetically sealed via holes.
It is a further object of the invention to provide a glass-ceramic structure with holes filled with a material of sufficient electrical conductivity.
It is another object of the invention to provide a glass-ceramic structure where said conductive material does not corrode the glass-ceramics.
It is also an object of the invention to provide a simple method for forming a glass-ceramic structure.
It is still a further object of the invention to provide a method achieving a hermetic sealing of the holes, where no excessive tensions due to different coefficients of expansion are created when the structure is heated.